This invention relates to an automatic focusing device in a microscope system which is used to inspect for example, integrated circuits (ICs).
In the automatic focusing operation of such a microscope system, a focusing detecting beam of infrared radiation is applied to a specimen mounted on a stage through a dichroic mirror and an objective which are arranged in the microscope system, and the stage is moved vertically in order to focus the microscope system. The focus is detected from the beam reflected from the specimen. There has been a strong demand for this automatic focusing function especially with the development of the IC industry. An automatic focusing device with an objective having a magnification not more than 40 is commercially available. However, an automatic focusing device is not available at all which has an objective having a magnification higher than 40, especially a magnification of 100. Accordingly, no automatic focusing device can be used for inspecting ICs and particularly LSIs which have very minute patterns.